Efficiency improvement of thermal power plants through specific entropy generation

Numerous studies have indicated that when neither the rate of heat input nor the power output in a thermal power plant is treated as a fixed parameter, minimizing the entropy generation does not lead to an improved thermal efficiency. This article presents a unified approach to resolve this issue by introducing specific entropy generation defined as the total entropy generation rate per unit flowrate of the fuel. A regenerative gas turbine and a combined cycle power plants are chosen for the purpose of discussion. It is found that the thermal efficiency inversely correlates with specific entropy generation, and minimization of specific entropy generation is identical to maximization of thermal efficiency. An illustrative example is presented to show how specific entropy generation can be applied to improve the efficiency of an integrated cycle. The results reveal that 85% of the inefficiencies of the combined cycle studied takes place in the gas turbine cycle. Recovering the thermal energy of the flue gases for both preheating the air and producing the steam within heat recovery steam generator yields 3.5 percentage points more efficiency than the case in which the heat of flue gases is only recovered for producing steam. With this modification, minimum specific entropy generation is dropped from 1489 to 1391 (J/K·mole fuel).